1. Field of the Invention
This invention relates to a process for producing a printed circuit board. More specifically, it relates to a process for producing a copper through-hole printed circuit board characterized in that an organic compound containing in the molecule both a long hydrophobic group portion and a polar group portion having strong affinity for copper metal is coated as an etching resist instead of applying electrolytic solder plating as in the prior art.
In the production of a copper through-hole printed circuit board, it is necessary to print a circuit accurately on both surfaces of the circuit board and connect the circuits on both surfaces accurately at many required parts. The present invention provides a process for producing a copper through-hole printed circuit board having high reliability at low cost within a short period of time.
2. Detailed of the Prior Art
A method known as the hole-filling method has gained most widespread use in the past for the production of copper through-hole printed circuit boards. It comprises providing many holes at required parts of a laminated plate copper-clad on both surfaces, treating the resulting base board having many holes with a catalyst, subjecting it to copper electroless plating, then subjecting it to copper electroplating to form a copper coating throughout the base board including the holes, thereafter filling a hole-filling ink into all the holes to prevent entry of an etching solution and protect the copper coating in the holes, printing an etching resist through a positive so as to give required circuits to both surfaces of the base board, and thereafter etching the base board to remove the copper from the unwanted exposed parts while leaving those parts which are protected by the etching resist and the hole-filling ink. As a result, a copper through-hole printed circuit board is formed.
According to this hole-filling method, the hole-filling ink may shrink during the process by reason of evaporation of the solvent from the ink, for example, so that the etching solution is likely to get into the holes and dissolve the copper coating. Furthermore, as a result of this shrinking, the thickness of the hole-filling ink layer tends to become smaller than the thickness of the printed circuit board. For this reason, it is very likely that the copper coating at edge parts between the base board and the holes being dissolved (defect of edge). Hence, products of high reliability cannot be obtained.
The electrolytic solder plating method (solder through-hole method) is known for removing the aforesaid defects of the hole-filling method and obtaining products of high reliability. This method is the same as the hole-filling method up to the copper electroplating step, but differs from the latter in that a resist ink is printed through a negative image. This resist ink is not an etching resist, but is a resist for the electrolytic solder plating to be subsequently carried out. Needless to say, this method does not require the use of the hole-filling ink. As a result of printing the resist ink through the negative, copper is exposed at al circuits and holes required for printed wiring, and those parts which are unwanted for printed wiring are protected with the plating resist ink. The exposed parts are subjected to electrolytic solder plating, and thereafter the plating resist ink printed through the negative is removed. Since the solder has resistance to etching, it acts as an etching resist and protects copper.
Since this method does not use the hole-filling ink, there is no risk of removal of the copper coating in the holes, particularly in the angular parts between the printed circuit and the holes, and it gives products of high reliability.
As an improvement over the electrolytic solder plating method, a method has also been proposed in which a resist ink is printed through a negative on a base board subjected to electroless copper plating, and thereafter, the base board is subjected to electrolytic copper plating and electrolytic solder plating. This method can given products comparable to those obtained by the electrolytic solder plating method. Furthermore, since the resist ink is printed in advance over an area which requires electroless copper plating, it has the advantage of reducing the time and cost over the aforesaid method which involves copper electroplating on all over the surface of the base board.
Thus, the electrolytic solder plating method is excellent in that it can produce a through-hole printed circuit board having high reliability, but has the defect that it requires a longer period of time and larger expenses for production than the hole-filling method. In particular, it has recently been required to provide large amounts of products at low cost within short periods of time. Accordingly, by using the electrolytic solder plating method, it is impossible to meet the consumers' demands fully. Furthermore, according to the electrolytic solder plating method, hydrofluoric acid or lead used may cause pollution, and large expenditures must go into countermeasures against pollution.
It has been strongly desired therefore to develop a process for producing a through-hole printed circuit board having reliability equivalent to, or higher than, that attained by the electrolytic solder plating method, which furthermore is simple in process steps, permits a shortened period of time and a reduced cost of production, and enables simplified treatment of the waste liquor for pollution control.